Various systems have been developed for measuring the surface flatness of a work piece. Applications for such systems cover a wide range of products on which a flat surface is desired. One system which is employed in the steel sheet industry uses mechanical scanning. In this system a single transducer or a small number of transducers, such as a laser distance sensor or a capacitance sensor is moved over the sheet surface to provide an output indicative of variations in the flatness of the sheet. Systems of this type require considerable skill to operate in order to meet the necessary standards of accuracy required.
Other systems measure surface shape by the use of optical flats. Optical flat techniques are suitable, however, only when the surface being checked for flatness has relatively highly polished or light-reflective characteristic sufficient to produce the desired optical pattern. Optical flats systems are highly sensitive, are typically useful only for relatively small area surfaces, and are very susceptible to error, particularly when the system is operated by persons having relatively little familiarity with the system.
For machine shop operations, systems have been developed for measuring the flatness of a relatively small area work piece at different stages during the machining operation. A patent directed to such a system is Boettcher U.S. Pat. No. 3,314,328. The small area flat surface to be measured is placed on a transparent sheet, which has grating lines formed on its surface. A collimated light source then is directed to the specimen under inspection through the transparent support surface. The light is transmitted through the support surface at an angle, and is reflected onto a mirror, which in turn reflects the light to an observation window. The grating pattern is observed in the window; and if there are imperfections in the flatness of the surface under test, those imperfections show up as curved lines in the observed pattern. Because collimated light is required, and, further, because of the relatively complex transmission path, the system is of no use with large area surfaces such as rolled steel or aluminum sheets.
Two other systems, which are somewhat similar to the system of Boettcher, and which use collimated light for flatness measurements, are disclosed in the patents to Goddard U.S. Pat. No. 2,867,149 and Jaerisch U.S. Pat. No. 3,858,981. The system disclosed in the Goddard patent is substantially the same as the one described above for the Boettcher patent. The device in Jaerisch is based upon a reflective object, and is substantially a two-beam interferometer with an advantage over standard interferometers in that the grating produces an incident light beam at an angle of incidence to reduce the sensitivity. The systems of all three of these patents measure surface height or surface variations from absolute flatness.
In each of the devices disclosed in Goddard, Boettcher and Jaerisch patents, the illumination system requires collimated light. To achieve parallel illumination (namely collimated light), the light source must be small in extent, producing a fan of rays diverging from the given point, with a lens used to "collimate" the light. This causes the light rays to be parallel when the source is placed at the rear focal point of the lens, in effect placing the source at infinity. The size of the collimated light bundle is therefore defined by the diameter or physical size of the collimating lens. Obviously, collimated systems of this type are limited for use with relatively small area surfaces defined by the diameter of the collimating lens.
In each of the systems disclosed in the Boettcher, Goddard and Jaerisch patents, the grating is in close proximity to the object under test. The grating consists of alternating clear and opaque straight lines. Collimated (parallel) light rays falling on the grating cast a shadow of the lines on the surface of the object under test. The requirement of the systems of these patents is that the shadow of the grating must be well defined on the object under test. If the grating lines are narrowly spaced, the distance between the grating and the object under test must be small, due to diffraction of the grating lines blurring the shadow. For grating lines spaced farther apart, the object under test can be slightly farther away from the grating. Typically, the spacing between the grating and the object under test must be less than the square of the grating line spacing. Consequently, for a 100 line per millimeter grating, the object under test cannot be farther away than approximately 100 microns. In the systems of the Boettcher, Goddard and Jaerisch patents, the grating and collimated bundle of light are approximately the same size as the test object or surface undergoing measurement. This is a consequence of the fact that the systems directly measure the surface height.
A different approach to determining variations in surface height or, therefore, departures from surface flatness is disclosed in the Suzuki U.S. Pat. No. 4,102,578. This patent does not employ shadows or grating, but instead images a single line on the surface under test. This line is imaged through a grating and onto a moving film to create a full image. Instead of building up an image point-by-point, the system of Suzuki generates an image one line at a time. The device of Suzuki measures surface height directly; and the sensitivity to height is determined primarily by the angle between the projector and the imaging arm.
Another approach is described in the system of the patent to Quinn U.S. Pat. No. 4,390,277. The instrument of the Quinn patent uses information of local surface slope variation to determine, over a small region at a time, the RMS variation of a surface over this small region. Collimated light is required; and the slope information is not used to determine the height variation of the surface. Instead, it is used to determine the scattering properties. The instrument must scan the surface to measure the complete surface. This is a point-by-point scanning system, again, useful only for relatively small area surfaces.
At the present time rolled aluminum and steel sheet and plate products are produced in widths that vary typically between 36" to 83". These products are used for lithography, beverage cans, foils and closures, as well as for panels in household appliances and automobiles. Plate products are typically sold in the specified lengths without coiling. The flatness of rolled sheet and plate products is an important attribute from a customer satisfaction standpoint, as well as from the production standpoint. As a result, flatness is monitored at various stages during fabrication. In the cold mills and the finishing mills sections of rolled sheet are cut to lengths of 60" to 120" and placed on an inspection table. The flatness of the sample is then measured either manually or through the use of single point measurement devices that scan over the entire area. The manual measurement of flatness is difficult and inaccurate, while the scanning devices are time consuming and not well suited for the production environment.
It is desirable to provide a sheet flatness measuring system and method, which does not require collimated light, which does not require a grating in close proximity to the object under test, which does not need to scan point-by-point or line-by-line to acquire information in a piece-wise fashion, and which further is capable of measuring the flatness of large sheets having reflective and semi-reflective surfaces.